1. Field of the Invention
The present invention relates to long barrel firearms that use gas cycling systems.
2. Description of the Related Art
In long-rifle firearms that use gas cycling systems, a gas port is positioned between the muzzle and the chamber of the barrel. Expanding gas resulting from discharge of a cartridge through the barrel bore flows through the gas port, where it is directed by a gas flowpath to a piston system or direct impingement that ejects the spent cartridge and chambers a new cartridge.
With precision firearms, however, ejection/expulsion of the expanding gas through the gas port causes an opposing force that affects harmonics of the barrel. Because this downward force is distal from the center of mass of the weapon, the magnitude of the torque caused by the force is sufficient to cause a deflection of the muzzle that unpredictably affects accuracy of the projectile. Specifically, the barrel tip displacement Zt can be approximated asZt=(FP2/6EI)(3L−P),  (1)where F is the force at the gas port caused by the ejecting gas, P is the distance between the breech and the port, E is the modulus of elasticity (2.9×107 psi) of common barrel steels, I is the moment of inertia of the barrel, and L is the barrel length along the barrel axis between the breech and the tip. The moment of inertia, I, may be approximated asI=π(Bodb4−Bidb4)/64,  (2)where Bodb is the barrel outer diameter average and Bidb is the barrel bore diameter. The force F at the gas port may be calculated asF=Sπ(D/2)2,  (3)where S is the gas port pressure and D is the port diameter. The required gas flowpath length G may be approximated asG=Vt/π(Bidt/2)2),  (4)where Vt is the volume of the gas flowpath and Bidt is the gas tube inner diameter